Organic light-emitting diode (OLED) display

ABSTRACT

An organic light-emitting diode (OLED) display is disclosed. In one aspect, the OLED display includes a plurality of pixels, each including a driving thin film transistor (TFT) formed over a substrate and including a driving gate electrode, a first storage capacitor comprising a first electrode and a second electrode, and a second storage capacitor comprising a third electrode and a fourth electrode. The first electrode is electrically connected to the driving gate electrode and the second electrode is formed over the first electrode and electrically insulated from the first electrode. The third electrode is electrically connected to the first electrode, is formed on a different layer from each of the first and second electrodes, and does not overlap the second electrode. The fourth electrode is formed over the third electrode and electrically insulated from the third electrode.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/465,683, filed Aug. 21, 2014, which claims priority under 35 U.S.C.119 of Korean Patent Application No. 10-2014-0027429, filed on Mar. 7,2014, in the Korean Intellectual Property Office, the disclosures ofwhich are incorporated by reference herein in their entireties.

BACKGROUND Field

The described technology generally relates to an organic light-emittingdiode (OLED) display.

Description of the Related Technology

Organic light-emitting diode (OLED) displays include two electrodes andan organic emission layer interposed therebetween. Electrons injectedfrom one of the electrodes and holes injected from the other electrodeare combined in the organic emission layer to generate excitons. Whenthe excitons fall from an excited state to a ground state they emitlight.

OLED displays include a plurality of pixels each including an OLED whichis a self-emissive component. Each of the pixels also includes aplurality of thin film transistors (TFTs) and at least one capacitor.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

One inventive aspect is an organic light-emitting diode (OLED) displayincluding at least one capacitor in a pixel.

Another aspect is an OLED display including a capacitor structure formaintaining and supplementing capacitance in pixels having ahigh-resolution.

Another aspect is an OLED display including a plurality of pixels. Eachof the pixels includes a driving thin film transistor (TFT) provided ona substrate; a first storage capacitor including a first electrode and asecond electrode, in which the first electrode is connected to a drivinggate electrode of the driving TFT and the second electrode is providedon the first electrode and insulated from the first electrode; and asecond storage capacitor including a third electrode and a fourthelectrode, in which the third electrode is electrically connected to thefirst electrode, provided on a different layer from the first and secondelectrodes, and does not overlap the second electrode of the firststorage capacitor, and the fourth electrode is provided on the thirdelectrode and insulated from the third electrode.

The driving gate electrode and the first electrode may be integrallyformed on the same layer.

The driving TFT may further include a driving semiconductor layer thatis provided below the driving gate electrode and is insulated from thedriving gate electrode by a first gate insulating layer and a secondgate insulating layer that are sequentially stacked on the substrate.

The OLED display may further include an emission control TFT that isprovided between the driving TFT and an OLED, and includes an emissioncontrol semiconductor layer that is on the same layer as the drivingsemiconductor layer of the driving TFT; and an emission control gateelectrode that is insulated from the emission control semiconductorlayer by the first gate insulating layer that covers the emissioncontrol semiconductor layer.

The OLED display may further include a third gate insulating layerprovided between the first and second electrodes.

The OLED display may further include a first interlayer insulating layerprovided between the second and third electrodes, and in which the firstand third electrodes are electrically connected to each other via acontact hole that is commonly formed in the third gate insulating layerand the first interlayer insulating layer.

The OLED display may further include a second interlayer insulatinglayer provided between the third and fourth electrodes.

The second electrode may be connected to a driving voltage line thattransmits a driving voltage for driving the OLED.

The fourth electrode may be connected to an initialization voltage linethat transmits an initialization voltage for initializing the drivingTFT.

The OLED display may further include a protection layer provided on thefourth electrode. The OLED may include an anode electrode that isprovided on the protection layer, a cathode electrode facing the anodeelectrode, and an intermediate layer formed between the anode andcathode electrodes.

Another aspect is an OLED display including a plurality of pixels. Eachof the pixels includes: a driving thin film transistor (TFT) provided ona substrate; a first storage capacitor including a first electrode and asecond electrode, in which the first electrode is connected to a drivinggate electrode of the driving TFT and the second electrode is providedon the first electrode and insulated from the first electrode; and asecond storage capacitor, which does not overlap the first storagecapacitor, including a third electrode and a fourth electrode, in whichthe third electrode is electrically connected to the first electrode,and the fourth electrode is provided below the third electrode,insulated from the third electrode, and is provided on the same layer asthe first electrode.

The driving gate electrode and the first electrode may be integrallyformed on the same layer.

The driving TFT may further include a driving semiconductor layer thatis provided below the driving gate electrode and is insulated from thedriving gate electrode by a first gate insulating layer.

The OLED display may further include an emission control TFT that isprovided between the driving TFT and an OLED, and includes an emissioncontrol semiconductor layer that is on the same layer as the drivingsemiconductor layer of the driving TFT; and an emission control gateelectrode that is insulated from the emission control semiconductorlayer by the first gate insulating layer that covers the emissioncontrol semiconductor layer, and is provided on the same layer as thedriving gate electrode.

The OLED display may further include a second gate insulating layerprovided between the first and second electrodes.

The OLED display may further include a first interlayer insulatinglayer, which is provided between the third and fourth electrodes.

The second electrode may be connected to a driving voltage line thattransmits a driving voltage for driving the OLED.

The OLED display may further include an initialization TFT that isprovided between the driving TFT and an initialization voltage line thattransmits an initialization voltage, turned on by a scan signal, andthus transmits the initialization voltage to a driving gate electrode.

The fourth electrode may be connected to the scan line that transmitsthe scan signal.

The OLED display may further include a protection layer provided on thefourth electrode. The OLED may include an anode electrode that isprovided on the protection layer, a cathode electrode facing the anodeelectrode, and an intermediate layer formed between the anode andcathode electrodes.

Another aspect is an OLED display, comprising a plurality of pixels,wherein each of the pixels comprises: a driving thin film transistor(TFT) formed over a substrate and including a driving gate electrode; afirst storage capacitor comprising a first electrode and a secondelectrode, wherein the first electrode is electrically connected to thedriving gate electrode and wherein the second electrode is formed overthe first electrode and electrically insulated from the first electrode;and a second storage capacitor comprising a third electrode and a fourthelectrode, wherein the third electrode is electrically connected to thefirst electrode, is formed on a different layer from each of the firstand second electrodes, and does not overlap the second electrode andwherein the fourth electrode is formed over the third electrode andelectrically insulated from the third electrode.

Each of the driving gate electrodes can be integrally formed with thecorresponding first electrode. Each of the driving TFTs can furthercomprise a driving semiconductor layer formed under the driving gateelectrode, wherein each of the driving semiconductor layers iselectrically insulated from the corresponding driving gate electrode viaa first gate insulating layer and a second gate insulating layer thatare sequentially stacked over the substrate. Each of the pixels canfurther comprise an OLED and an emission control TFT that is formedbetween the driving TFT and the OLED, wherein each of the emissioncontrol TFTs comprises an emission control semiconductor layer formed onthe same layer as the driving semiconductor layer and an emissioncontrol gate electrode electrically insulated from the emission controlsemiconductor layer via the first gate insulating layer. The OLEDdisplay can further comprise a third gate insulating layer formedbetween each of the first and second electrodes. The OLED display canfurther comprise a first interlayer insulating layer formed between eachof the second and third electrodes, wherein each of the first electrodesis electrically connected to the corresponding third electrode via acorresponding contact hole formed in each of the third gate insulatinglayer and the first interlayer insulating layer.

The OLED display can further comprise a second interlayer insulatinglayer formed between each of the third and fourth electrodes. Each ofthe second electrodes can be electrically connected to a correspondingdriving voltage line configured to transmit a driving voltage fordriving the corresponding OLED. Each of the fourth electrodes can beelectrically connected to a corresponding initialization voltage lineconfigured to transmit an initialization voltage for initializing thecorresponding driving TFT. The OLED display can further comprise aprotection layer formed over the fourth electrode, wherein each of theOLEDs comprises an anode electrode formed over the protection layer; acathode electrode facing the anode electrode; and an intermediate layerinterposed between the anode and cathode electrodes.

Another aspect is an OLED display, comprising a plurality of pixels,wherein each of the pixels comprises: a driving thin film transistor(TFT) formed over a substrate and including a driving gate electrode; afirst storage capacitor comprising a first electrode and a secondelectrode, wherein the first electrode is electrically connected to thedriving gate electrode and wherein the second electrode is formed overthe first electrode and electrically insulated from the first electrode;and a second storage capacitor comprising a third electrode and a fourthelectrode, wherein the second storage capacitor does not overlap thefirst storage capacitor, wherein the third electrode is electricallyconnected to the first electrode, and wherein the fourth electrode is:formed under the third electrode, electrically insulated from the thirdelectrode, and formed on the same layer as the first electrode.

Each of the driving gate electrodes can be integrally formed with thecorresponding first electrode. Each of the driving TFTs can furthercomprise a driving semiconductor layer formed under the driving gateelectrode, wherein each of the driving semiconductor layers iselectrically insulated from the corresponding driving gate electrode viaa first gate insulating layer. Each of the pixels can further comprisean OLED and an emission control TFT formed between the driving TFT andthe OLED, wherein each of the emission control TFTs comprises anemission control semiconductor layer formed on the same layer as thedriving semiconductor layer and an emission control gate electrodeelectrically insulated from the emission control semiconductor layer viathe first gate insulating layer, wherein the emission control gateelectrode is formed on the same layer as the driving gate electrode. TheOLED display can further comprise a second gate insulating layer formedbetween the first and second electrodes. The OLED display can furthercomprise an interlayer insulating layer formed between the third andfourth electrodes.

Each of the second electrodes can be electrically connected to acorresponding driving voltage line configured to transmit a drivingvoltage for driving the corresponding OLED. Each pixel can furthercomprise an initialization TFT formed between the driving TFT and aninitialization voltage line configured to transmit an initializationvoltage, wherein each of the initialization TFTs is configured to beturned on by a corresponding scan signal so as to transmit theinitialization voltage to the corresponding driving gate electrode. Thefourth electrode can be electrically connected to a scan line configuredto transmit the scan signal. The OLED display can further comprise aprotection layer formed over the fourth electrode, wherein each of theOLEDs comprises an anode electrode formed over the protection layer; acathode electrode facing the anode electrode; and an intermediate layerinterposed between the anode and cathode electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an OLED display according to anembodiment.

FIG. 2 is an equivalent circuit diagram of a pixel of the OLED displayaccording to an embodiment.

FIGS. 3A and 3B are schematic plan views of the pixel of FIG. 2.

FIG. 4 is a cross-sectional view taken along lines A-A′ and B-B′ of FIG.3A.

FIG. 5 is an equivalent circuit diagram of a pixel of an OLED displayaccording to another embodiment.

FIGS. 6A and 6B are schematic plan views of the pixel of FIG. 5.

FIG. 7 is a cross-sectional view taken along lines C-C′, D-D′, and E-E′of FIG. 6A.

DETAILED DESCRIPTION OF CERTAIN INVENTIVE EMBODIMENTS

Capacitors used in the pixels of an OLED display typically include alower electrode, an upper electrode, and a dielectric material providedbetween the lower and upper electrodes. The capacitance of such acapacitor is proportionate to the overlapping area of the lower andupper electrodes. Accordingly, as the overlapping area of the lower andupper electrode decreases, the capacitance of the capacitor alsodecreases. As the resolution of OLED displays increases, more pixels arerequired to be included per unit area, reducing the available area foreach pixel. In order to normally operate, OLED displays are required toinclude a predetermined number of capacitors. Accordingly, capacitorsneed a new design that is appropriate for high-resolution OLED displays.

Reference will now be made in detail to embodiments, examples of whichare illustrated in the accompanying drawings, wherein like referencenumerals refer to like elements throughout. In this regard, the presentembodiments may have different forms and should not be construed asbeing limited to the descriptions set forth herein. Accordingly, theembodiments are merely described below, by referring to the figures, toexplain aspects of the present description. As used herein, the term“and/or” includes any and all combinations of one or more of theassociated listed items. Expressions such as “at least one of,” whenpreceding a list of elements, modify the entire list of elements and donot modify the individual elements of the list.

It will be understood that when a layer, region, or component isreferred to as being “formed on,” another layer, region, or component,it can be directly or indirectly formed on the other layer, region, orcomponent. That is, for example, intervening layers, regions, orcomponents may be present.

The sizes of elements in the drawings may be exaggerated for convenienceof explanation. In other words, since the sizes and thicknesses ofcomponents in the drawings may be exaggerated, the following embodimentsare not limited thereto.

FIG. 1 is a schematic block diagram of an OLED display 1000 according toan embodiment.

The OLED display 1000 includes a display unit 10 including a pluralityof pixels 1, a scan driver 20, a data driver 30, an emission controldriver 40, and a controller 50.

The display unit 10 is located at the intersection between a pluralityof scan lines SL1 to SLn+1, a plurality of data lines DL1 to DLm, and aplurality of emission control lines EL1 to ELn. The display unit 10includes the pixels 1 that are arranged in a matrix. The scan lines SL1to SLn+1 and the emission control lines EL1 to ELn extend in a seconddirection, i.e., a row direction and the data lines DL1 to DLm anddriving voltage lines ELVDDL extend in a first direction, i.e., a columndirection. In each pixel line, the n-value of the scan lines SL1 toSLn+1 may differ from the n-value of the emission control lines EL1 toELn.

Each pixel 1 is connected to three scan lines from among the scan linesSL1 to SLn+1 that extend across the display unit 10. The scan driver 20transmits three scan signals to each pixel 1 via the plurality of scanlines SL1 to SLn+1. That is, the scan driver 20 sequentially suppliesscan signals via first scan lines SL2 to SLn, second scan lines SL1 toSLn−1, or third scan lines SL3 to SLn+1.

Initialization voltage lines IL receive an initialization voltage forinitializing the display unit 10 from an external power supply sourceVINT.

Each pixel 1 is also connected to one of the data lines DL1 to DLm thatextend across the display unit 10 and one of the emission control linesEL1 to ELn that extend across the display unit 10.

The data driver 30 transmits data signals to the pixels 1 via the datalines DL1 to DLm. The data signals are supplied to a pixel selected by ascan signal whenever the scan signals are supplied via the first scanlines SL2 to SLn+1.

The emission control driver 40 generates and transmits emission controlsignals to the pixels 1 via the emission control lines EL1 to ELn. Theemission control signals control the emission timing of the pixels 1.The emission control driver 40 may be omitted depending on the internalstructure of the pixels 1.

The controller 50 converts a plurality of image signals R, G, and B thatare received from an external source into a plurality of image datasignals DR, DG, and DB and transmits the image data signals DR, DG, andDB to the data driver 30. Also, the controller 50 receives verticalsynchronization signals Vsync, horizontal synchronization signals Hsync,and clock signals MCLK, and generates and transmits control signals forcontrolling the operation of the scan driver 20, the data driver 30, andthe emission control driver 40. That is, the controller 50 generatesscan driver control signals SCS for controlling the scan driver 20, datadriver control signals DCS for controlling the data driver 30, andemission driver control signals ECS for controlling the emission controldriver 40, and transmits each signal to its corresponding driver.

Each of the pixels 1 receives a first power voltage ELVDD and a secondpower voltage ELVSS. The first power voltage ELVDD may be apredetermined high level voltage and the second power voltage ELVSS maybe a voltage that is lower than the first power voltage ELVDD or aground voltage. The first power voltage ELVDD is provided to the pixels1 via the driving voltage lines ELVDDL.

Each of the pixels 1 emits light having a predetermined brightness basedon a driving current that is supplied to OLEDs according to the datasignals received via the data lines DL1 to DLm.

FIG. 2 is an equivalent circuit diagram of the pixel 1 of the OLEDdisplay 1000 according to an embodiment.

The pixel 1 of the OLED display 1000 includes a pixel circuit 2 thatincludes a plurality of thin film transistors (TFTs) T1 to T7 and twostorage capacitors, namely, first and second storage capacitors Cst1 andCst2. The pixel 1 also includes an OLED that receives a driving currentvia the pixel circuit 2 and thus emits light.

The TFTs T1 to T7 include a driving TFT T1, a switching TFT T2, acompensation TFT T3, a first initialization TFT T4, a first emissioncontrol TFT T5, a second emission control TFT T6, and a secondinitialization TFT T7.

The pixel 1 includes a first scan line 14 that transmits a first scansignal Sn to the switching TFT T2 and the compensation TFT T3, a secondscan line 24 that transmits a second scan signal Sn−1 to the firstinitialization TFT T4, and a third scan line 34 that transmits a thirdscan signal Sn+1 to the second initialization TFT T7. The pixel 1 alsoincludes an emission control line 15 that transmits an emission controlsignal En to the first emission control TFT T5 and the second emissioncontrol TFT T6, a data line 16 that intersects the first scan line 14and transmits a data signal Dm, a driving voltage line 26 that transmitsthe first power voltage ELVDD, and an initialization voltage line 20that transmits an initialization voltage VINT to initialize the drivingTFT T1.

A gate electrode G1 of the driving TFT T1 is connected to a firstelectrode C1 of the first storage capacitor Cst1 and a third electrodeC3 of the second storage capacitor Cst2. A source electrode S1 of thedriving TFT T1 is connected to the driving voltage line 26 via the firstemission control TFT T5. A drain electrode D1 of the driving TFT T1 iselectrically connected to an anode electrode of the OLED via the secondemission control TFT T6. The driving TFT T1 receives the data signal Dmbased on a switching operation of the switching TFT T2 and supplies adriving current Ioled to the OLED.

A gate electrode G2 of the switching TFT T2 is connected to the firstscan line 14. A source electrode S2 of the switching TFT T2 is connectedto the data line 16. A drain electrode D2 of the switching TFT T2 isconnected to the source electrode S1 of the driving TFT T1 and to thedriving voltage line 26 via the first emission control TFT T5. Theswitching TFT T2 is turned on based on the first scan signal Sn receivedvia the first scan line 14 and performs a switching operation in whichthe data signal Dm received via the data line 16 is transmitted to thesource electrode S1 of the driving TFT T1.

A gate electrode G3 of the compensation TFT T3 is connected to the firstscan line 14. A source electrode S3 of the compensation TFT T3 isconnected to the drain electrode D1 of the driving TFT T1 and to theanode electrode of the OLED via the second emission control TFT T6. Adrain electrode D3 of the compensation TFT T3 is connected to the firstelectrode C1 of the first storage capacitor Cst1, the third electrode C3of the second storage capacitor Cst2, a drain electrode D4 of the firstinitialization TFT T4, and the gate electrode G1 of the driving TFT T1.The compensation TFT T3 is turned on based on the first scan signal Snreceived via the first scan line 14 and connects the driving TFT T1 toform a diode connection by connecting the gate electrode G1 and thedrain electrode D1 of the driving TFT T1.

A gate electrode G4 of the first initialization TFT T4 is connected tothe second scan line 24. A source electrode S4 of the firstinitialization TFT T4 is connected to the initialization voltage line20. The drain electrode D4 of the first initialization TFT T4 isconnected to the first electrode C1 of the first storage capacitor Cst1,the third electrode C3 of the second storage capacitor Cst2, the drainelectrode D3 of the compensation TFT T3, and the gate electrode G1 ofthe driving TFT T1. The first initialization TFT T4 is turned on basedon the second scan signal Sn−1 received via the second scan line 24 andperforms an initialization operation in which the initialization voltageVINT is transmitted to the gate electrode G1 of the driving TFT T1 andthus initializes the voltage of the gate electrode G1 of the driving TFTT1.

A gate electrode G5 of the first emission control TFT T5 is connected tothe emission control line 15. A source electrode S5 of the firstemission control TFT T5 is connected to the driving voltage line 26. Adrain electrode D5 of the first emission control TFT T5 is connected tothe source electrode S1 of the driving TFT T1 and the drain electrode D2of the switching TFT T2.

A gate electrode G6 of the second emission control TFT T6 is connectedto the emission control line 15. A source electrode S6 of the secondemission control TFT T6 is connected to the drain electrode D1 of thedriving TFT T1 and the source electrode S3 of the compensation TFT T3. Adrain electrode D6 of the second emission control TFT T6 is electricallyconnected to the anode electrode of the OLED. The first and secondemission control TFTs T5 and T6 are substantially simultaneously turnedon based on the emission control signal En received via the emissioncontrol line 15. Then, the first power voltage ELVDD is transmitted tothe OLED, and thus, the driving current Ioled flows in the OLED.

A gate electrode G7 of the second initialization TFT T7 is connected tothe third scan line 34. A source electrode S7 of the secondinitialization TFT T7 is connected to the anode electrode of the OLED. Adrain electrode D7 of the second initialization TFT T7 is connected tothe initialization voltage line 20. The second initialization TFT T7 isturned on based on the third scan signal Sn+1 received via the thirdscan line 34 and initializes the anode electrode of the OLED.

A second electrode C2 of the first storage capacitor Cst1 is connectedto the driving voltage line 26. The first electrode C1 of the firststorage capacitor Cst1 is connected to the gate electrode G1 of thedriving TFT T1, the third electrode C3 of the second storage capacitorCst2, the drain electrode D3 of the compensation TFT T3, and the drainelectrode D4 of the first initialization TFT T4.

The third electrode C3 of the second storage capacitor Cst2 is connectedto the gate electrode G1 of the driving TFT T1, the first electrode C1of the first storage capacitor Cst1, the drain electrode D3 of thecompensation TFT T3, and the drain electrode D4 of the firstinitialization TFT T4. A fourth electrode C4 of the second storagecapacitor Cst2 is connected to the initialization voltage line 20.

The second power voltage ELVSS is applied to a cathode electrode of theOLED. The OLED displays an image by receiving the driving current Ioledfrom the driving TFT T1 and thus emits light.

FIGS. 3A and 3B are schematic plan views of the pixel 1 of FIG. 2,according to an embodiment. FIGS. 3A and 3B each illustrate two pixels 1that are adjacent to each other (hereinafter, referred to as “twoadjacent pixels 1”).

Referring FIG. 3A, the pixel 1 includes the driving TFT T1, theswitching TFT T2, the compensation TFT T3, the first initialization TFTT4, the first emission control TFT T5, the second emission control TFTT6, the second initialization TFT T7, the first storage capacitor Cst1,and the second storage capacitor Cst2. The OLED is omitted from theillustration of FIG. 3A.

The driving TFT T1 includes a driving semiconductor layer A1, a drivinggate electrode G1, a driving source electrode S1, and a driving drainelectrode D1. The driving source electrode S1 is a driving source areain the driving semiconductor layer A1 which is doped with impurities andthe driving drain electrode D1 is a driving drain area in the drivingsemiconductor layer A1 which is doped with impurities. The driving gateelectrode G1 is connected to the first electrode C1 of the first storagecapacitor Cst1, the third electrode C3 of the second storage capacitorCst2, a compensation drain electrode D3 of the compensation TFT T3, anda first initialization drain electrode D4 of the first initializationTFT T4. In the FIG. 3A embodiment, the driving gate electrode G1 and thefirst electrode C1 are integrally formed. Also, the driving gateelectrode G1 and the third electrode C3 are connected via a firstcontact hole 51. In order to connect the driving gate electrode G1 andthe third electrode C3 via the first contact hole 51, the secondelectrode C2 does not completely overlap the first electrode C1, butincludes an opening OP that exposes the first electrode C1. The drivinggate electrode G1, the compensation drain electrode D3, and the firstinitialization drain electrode D4 are connected via the third electrodeC3 formed in the first contact hole 51 and a second contact hole 52.

The switching TFT T2 includes a switching semiconductor layer A2, aswitching gate electrode G2, a switching source electrode S2, and aswitching drain electrode D2. The switching source electrode S2 is aswitching source area in the switching semiconductor layer A2 which isdoped with impurities and the switching drain electrode D2 is aswitching drain area in the switching semiconductor layer A2 which isdoped with impurities. The switching source electrode S2 is connected tothe data line 16 via a third contact hole 53. The switching gateelectrode G2 is formed as a portion of the first scan line 14.

The compensation TFT T3 includes a compensation semiconductor layer A3,a compensation gate electrode G3, a compensation source electrode S3,and the compensation drain electrode D3. The compensation sourceelectrode S3 is a compensation source area in the compensationsemiconductor layer A3 which is doped with impurities and thecompensation drain electrode D3 is a compensation drain area in thecompensation semiconductor layer A3 which is doped with impurities. Thecompensation gate electrode G3 is formed as a portion of the first scanline 14.

The first initialization TFT T4 includes a first initializationsemiconductor layer A4, a first initialization gate electrode G4, afirst initialization source electrode S4, and the first initializationdrain electrode D4. The first initialization source electrode S4 is afirst initialization source area in the first initializationsemiconductor layer A4 which is doped with impurities and the firstinitialization drain electrode D4 is a first initialization drain areain the first initialization semiconductor layer A4 which is doped withimpurities. The first initialization source electrode S4 may beconnected to the initialization voltage line 20 by using a first contactmetal CM1 formed in a fourth contact hole 54. The first initializationdrain electrode D4 may be connected to the third electrode C3 of thesecond storage capacitor Cst2 via the second contact hole 52. The firstinitialization gate electrode G4 is formed as a portion of the secondscan line 24. A dual gate electrode is formed by overlapping the firstinitialization semiconductor layer A4 and the first initialization gateelectrode G4 twice.

The first emission control TFT T5 includes a first emission controlsemiconductor layer A5, a first emission control gate electrode G5, afirst emission control source electrode S5, and a first emission controldrain electrode D5. The first emission control source electrode S5 is afirst emission control source area in the first emission controlsemiconductor layer A5 which is doped with impurities and the firstemission control drain electrode D5 is a first emission control drainarea in the first emission control semiconductor layer A5 which is dopedwith impurities. The first emission control source electrode S5 may beconnected to the driving voltage line 26 via a fifth contact hole 55.The first emission control gate electrode G5 is formed as a portion ofthe emission control line 15.

The second emission control TFT T6 includes a second emission controlsemiconductor layer A6, a second emission control gate electrode G6, asecond emission control source electrode S6, and a second emissioncontrol drain electrode D6. The second emission control source electrodeS6 is a second emission control source area in the second emissioncontrol semiconductor layer A6 which is doped with impurities and thesecond emission control drain electrode D6 is a second emission controldrain area in the second emission control semiconductor layer A6 whichis doped with impurities. The second emission control drain electrode D6is connected to the anode electrode of the OLED by using a secondcontact metal CM2 formed through a sixth contact hole 56 and a via holeVIA, both of which are filled with the second contact metal CM2. Thesecond emission control gate electrode G6 is formed as a portion of theemission control line 15.

The second initialization TFT T7 includes a second initializationsemiconductor layer A7, a second initialization gate electrode G7, asecond initialization source electrode S7, and a second initializationdrain electrode D7. The second initialization source electrode S7 is asecond initialization source area in the second initializationsemiconductor layer A7 which is doped with impurities and the secondinitialization drain electrode D7 is a second initialization drain areain the second initialization semiconductor layer A7 which is doped withimpurities. The second initialization drain electrode D7 is connected tothe initialization voltage line 20 via the first contact metal CM1formed through the fourth contact hole 54. The second initializationsource electrode S7 is connected to the anode electrode of the OLED viathe second contact metal CM2 formed in the sixth contact hole 56 and athird contact metal CM3 that is connected to the second contact metalCM2. The second initialization gate electrode G7 is formed as a portionof the third scan line 34.

The first electrode C1 of the first storage capacitor Cst1 is directlyconnected to the driving gate electrode G1 and may be connected to thethird electrode C3 of the second storage capacitor Cst2 via the firstcontact hole 51. Also, the first electrode C1 of the first storagecapacitor Cst1 is connected to the first initialization TFT T4 and thecompensation TFT T3 via the third electrode C3 and the first and secondcontact holes 51 and 52. The first electrode C1 may have a floatingelectrode form and may overlap the driving semiconductor layer A1.

Although the second electrode C2 of the first storage capacitor Cst1overlaps the first electrode C1, the second electrode C2 does notcompletely cover the first electrode C1. The second electrode C2includes the opening OP that exposes a portion of the first electrodeC1. The opening OP may be the first contact hole 51. The secondelectrodes C2 that are formed in the two adjacent pixels 1 may beconnected to each other. The driving voltage line 26 may be connected tothe center of the second electrodes C2 that are formed in the twoadjacent pixels 1 via a seventh contact hole 57 and thus substantiallysimultaneously transmits the first power voltage ELVDD to the twoadjacent pixels 1. That is, the two adjacent pixels 1 receive thedriving voltage ELVDD from a single driving voltage line 26 by using thesecond electrodes C2 that are formed in the two adjacent pixels 1.

The third electrode C3 of the second storage capacitor Cst2 is connectedto the first electrode C1 of the first storage capacitor Cst1 via thefirst contact hole 51. Also, the third electrode C3 is connected to thecompensation TFT T3 and the first initialization TFT T4 via the secondcontact hole 52. The third electrode C3 and the data line 16 may beformed on the same layer and the third electrode C3 may have a floatingelectrode form. The third electrode C3 may overlap a portion of thesecond electrode C2 of the first storage capacitor Cst1, but does notoverlap the first electrode C1. Therefore, as illustrated in FIG. 3B,the second storage capacitor Cst2 may be separated from the firststorage capacitor Cst1, and the second storage capacitor Cst2 may have adifferent storage capacity than the storage capacity of the firststorage capacitor Cst1. Accordingly, it is possible to solve the problemof decreasing storage capacity due to increasing resolution in OLEDdisplays. Thus, the storage capacity in the pixel may be maintained at asimilar level to lower resolution displays.

The fourth electrode C4 of the second storage capacitor Cst2 overlapsthe third electrode C3. The fourth electrode C4 is connected to theinitialization voltage line 20 that is formed on the same layer as thefourth electrode C4. In particular, the initialization voltage line 20and the fourth electrode C4 may be formed integrally. Since theinitialization voltage line 20 is connected to the first and secondinitialization TFTs T4 and T7 via the fourth contact hole 54, the fourthelectrode C4 may also be connected to the first and secondinitialization TFTs T4 and T7.

The first to third scan lines 14, 24, and 34 and the emission controlline 15 are all formed on the same layer and extend in the seconddirection. The first to third scan lines 14, 24, and 34 and the emissioncontrol line 15 may be formed between semiconductor layers and firstgate electrodes such that the first to third scan lines 14, 24, and 34and the emission control line 15 are insulated from the semiconductorlayers and the first gate electrodes.

The data line 16 is formed on the same layer as the third electrode C3of the second storage capacitor Cst2 and extends in the first direction.

The driving voltage line 26 and the initialization voltage line 20 areformed on the same layer as the fourth electrode C4 of the secondstorage capacitor Cst2 and extend in the first direction.

The two adjacent pixels 1 share the driving voltage line 26. Inparticular, the driving voltage line 26 is formed between the twoadjacent pixels 1 and extends in the first direction. The drivingvoltage line 26 is connected to the first emission control TFT T5, whichis included in each of the two adjacent pixels 1, via the fifth contacthole 55, and to the second electrode C2 of the first storage capacitorCst1, which is commonly included in the two adjacent pixels 1, via theseventh contact hole 57. According to the present embodiment, the twoadjacent pixels 1 share the driving voltage line 26, and thus the twoadjacent pixels 1 may form a symmetrical structure about the drivingvoltage line 26. Accordingly, the number of driving voltage lines 26 maybe reduced, and thus, it is possible to obtain more design space due tothe reduction.

FIG. 4 is a cross-sectional view taken along lines A-A′ and B-B′ of FIG.3A. FIG. 4 illustrates the driving TFT T1 and the second emissioncontrol TFT T6 from among the plurality of TFTs, the first storagecapacitor Cst1, and the second storage capacitor Cst2.

Among elements, such as lines, electrodes, and semiconductor layers, ina cross-section taken along lines A-A′ and B-B′, elements that havelittle relation to the driving TFT T1, the second emission control TFTT6, the first storage capacitor Cst1, and the second storage capacitorCst2 are omitted in FIG. 4 so as to clarify features of the embodiments.Therefore, FIG. 4 may be different from an actual cross-sectional viewtaken along lines A-A′ and B-B′ of FIG. 3A.

Referring to FIG. 4, a buffer layer 101 is formed on a substrate 100.The buffer layer 101 functions as a barrier layer and/or blocking layerfor preventing impurity ions from spreading, blocking penetration ofmoisture or external air, and planarizing the surface of the substrate.

The driving semiconductor layer A1 of the driving TFT T1 and the secondemission control semiconductor layer A6 of the second emission controlTFT T6 are formed on the buffer layer 101. The driving and secondemission control semiconductor layers A1 and A6, which are formed ofpolysilicon, include a channel area that is not doped with impurities,and a source area and a drain area which are respectively formed on bothsides of the channel area and doped with impurities. In this case, theimpurities differ according to the type of TFT, and may be N-typeimpurities or P-type impurities. Although not illustrated, the drivingsemiconductor layer A1 and the second emission control semiconductorlayer A6 may simultaneously formed while being connected to theswitching semiconductor layer A2 of the switching TFT T2, thecompensation semiconductor layer A3 of the compensation TFT T3, thefirst initialization semiconductor layer A4 of the first initializationTFT T4, the first emission control semiconductor layer A5 of the firstemission control TFT T5, and the second initialization semiconductorlayer A7 of the second initialization TFT T7.

A first gate insulating layer GI1 is formed on the entire surface of thesubstrate 100 to cover the driving and second emission controlsemiconductor layers A1 and A6. The first gate insulating layer GI1 maybe a single layer or a plurality of layers formed of an inorganicmaterial such as silicon oxide or silicon nitride. The first gateinsulating layer GI1 insulates the semiconductor layers from the firstgate electrodes. According to an embodiment, the first gate insulatinglayer GI1 is thicker than a second gate insulating layer GI2 or a thirdgate insulating layer GI3 which will be described below. The first gateinsulating layer GI1 insulates the semiconductor layers of the switchingTFT T2, the compensation TFT T3, the first initialization TFT T4, thefirst emission control TFT T5, the second emission control TFT T6, andthe second initialization TFT T7 from the first gate electrodes. The“first gate electrodes” will be described in detail below. When thefirst gate insulating layer GI1 is thick, a parasitic capacitancebetween the semiconductor layers and the first gate electrodes may bereduced, and thus, stains may be reduced or removed from imagesdisplayed by the OLED display.

The second emission control gate electrode G6 of the second emissioncontrol TFT T6 is formed on the first gate insulating layer GI1. Also,although not illustrated, the switching gate electrode G2 of theswitching TFT T2, the compensation gate electrode G3 of the compensationTFT T3, the first initialization gate electrode G4 of the firstinitialization TFT T4, the second initialization gate electrode G7 ofthe second initialization TFT T7, and the first emission control gateelectrode G5 of the first emission control TFT T5 are simultaneouslyformed with the second emission control gate electrode G6. The “firstgate electrodes” as used in this embodiment refers to the switching gateelectrode G2, the compensation gate electrode G3, the firstinitialization gate electrode G4, the first emission control gateelectrode G5, the second emission control gate electrode G6, and thesecond initialization gate electrode G7 which are formed of a first gateline GL1 material. The first gate electrodes are defined bysemiconductor layers that overlap the first to third scan lines 14, 24,and 34 and the emission control line 15. Therefore, a process of formingthe first gate electrodes is thus a process of forming the first tothird scan lines 14, 24, and 34 and the emission control line 15. Thefirst gate line GL1 material may include at least one metal selectedfrom the group of aluminum (Al), platinum (Pt), palladium (Pd), silver(Ag), magnesium (Mg), gold (Au), nickel (Ni), neodymium (Nd), iridium(Ir), chromium (Cr), lithium (Li), calcium (Ca), molybdenum (Mo),titanium (Ti), tungsten (W), and copper (Cu).

The second gate insulating layer GI2 may be stacked on the entiresurface of the substrate 100 to cover the first gate electrodes. Thesecond gate insulating layer GI2 may be a single layer or a plurality oflayers formed of an inorganic material such as silicon oxide or siliconnitride. The second gate insulating layer GI2 insulates the first andsecond gate electrodes from each other. The “second gate electrodes”will be described in detail below. The second gate insulating layer GI2is thinner than the first gate insulating layer GI1.

The driving gate electrode G1 of the driving TFT T1 and the firstelectrode C1 of the first storage capacitor Cst1 connected thereto areformed on the second gate insulating layer GI2. The “second gateelectrodes” as used in this embodiment refers to the driving gateelectrode G1 and the first electrode C1 that are formed of a second gateline GL2 material. Similar to the first gate line GL1 material, thesecond gate line GL2 material may also include at least one metalselected from the group of Al, Pt, Pd, Ag, Mg, Au, Ni, Nd, Ir, Cr, Li,Ca, Mo, Ti, W, and Cu.

According to an embodiment, in the driving TFT T1, the first and secondgate insulating layers GI1 and GI2 are formed between the drivingsemiconductor layer A1 and the driving gate electrode G1 so that thedriving semiconductor layer A1 and the driving gate electrode G1 areinsulated from each other. Therefore, by forming a thick insulatinglayer between the driving semiconductor layer A1 and the driving gateelectrode G1, the parasitic capacitance between the drivingsemiconductor layer A1 and the driving gate electrode G1 is reduced.Accordingly, the gate voltage Vgs, which is applied to the driving gateelectrode G1, increases to a wider driving range, and the magnitude ofthe gate voltage Vgs may be changed so that light emitted from the OLEDhas a finer gradation in gray scales. That is, since the driving TFT T1has a wide driving range by forming a thick insulating layer between thedriving semiconductor layer A1 and the driving gate electrode G1, thedriving semiconductor layer A1 does not have to be formed relativelylong to increase the driving range of the driving TFT T1. Therefore, thedriving semiconductor layer A1 may be designed to be short, and thus,the entire size of the driving TFT T1, and more particularly, the sizeof the driving gate electrode G1 and the overall size occupied by theeach of the pixels can be reduced. Thus, the resolution of the OLEDdisplay can be increased.

The third gate insulating layer G13 is stacked on the entire surface ofthe substrate 100 to cover the second gate electrodes. The third gateinsulating layer G13 may be a single layer or a plurality of layersformed of an inorganic material such as silicon oxide or siliconnitride. The third gate insulating layer G13 insulates the second gateelectrodes and third gate electrode, and functions as a dielectric layerbetween the first and second electrodes C1 and C2 of the first storagecapacitor Cst1. The “third gate electrode” will be described in detailbelow. The third gate insulating layer G13 is thinner than the firstgate insulating layer GI1 so as to increase the storage capacity of thefirst storage capacitor Cst1.

The second electrode C2 of the first storage capacitor Cst1 is formed onthe third gate insulating layer G13. The “third gate electrode” as usedin this embodiment refers to the second electrode C2 that is formed of athird gate line GL3 material. Similar to the first and second gate linesGL1 and GL2 materials, the third gate line GL3 material may also includeat least one metal selected from the group of Al, Pt, Pd, Ag, Mg, Au,Ni, Nd, Ir, Cr, Li, Ca, Mo, Ti, W, and Cu.

According to an embodiment, the first storage capacitor Cst1 overlapsthe driving TFT T1. In particular, since the driving gate electrode G1and the first electrode C1 are integrally formed in some embodiments,the first storage capacitor Cst1 overlaps the driving TFT T1. However,as described above, when the size of the driving TFT T1, and moreparticularly, the size of the driving gate electrode G1, is reduced inorder to manufacture increase the resolution of the OLED display, thesize of the first electrode C1 is also reduced. Thus, the storagecapacity of the first storage capacitor Cst1 is reduced according toEquation 1 below. In Equation 1, “C” is the storage capacity of astorage capacitor, “ε” is the dielectric constant, “A” is the area theoverlap between the electrodes of the capacitor, and “d” is the distancebetween the overlapping electrodes.

$\begin{matrix}{C = {ɛ\frac{A}{d}}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

A substantially uniform storage capacity should be maintained in eachpixel so that the gray scales can be substantially consistentlydisplayed in the OLED display without any faults. Therefore, a designfor maintaining or supplementing the reduced storage capacity of thefirst storage capacitor Cst1 may be necessary. According to anembodiment, the second storage capacitor Cst2 is separately formed fromthe first storage capacitor Cst1 so as to maintain or supplement thestorage capacity of first storage capacitor Cst1 in each pixel.

A first interlayer insulating layer ILD1 is formed on the entire surfaceof the substrate 100 to cover the second electrode C2 of the firststorage capacitor Cst1. The first interlayer insulating layer ILD1 maybe formed of an organic insulating material, an inorganic insulatingmaterial, or have a multi-layer structure in which organic and inorganicinsulating materials are alternately formed. For example, the inorganicinsulating material may include a metal oxide or a metal nitride, suchas silicon oxide (SiO₂), silicon nitride (SiN_(x)), silicon oxynitride(SiON), aluminum oxide (Al₂O₃), titanium oxide (TiO₂), tantalum oxide(Ta₂O₅), hafnium oxide (HfO₂), or zinc oxide (ZnO₂). The firstinterlayer insulating layer ILD1 insulates the third gate electrode fromfirst data electrodes. The “first data electrodes” will be described indetail below.

The third electrode C3 of the second storage capacitor Cst2 is formed onthe first interlayer insulating layer ILD1. Although not illustrated,the data line 16, and the first and second contact metals CM1 and CM2are simultaneously formed with the third electrode C3. The thirdelectrode C3 is connected to the first electrode C1 via the firstcontact hole 51 formed in the third gate insulating layer GI3 and thefirst interlayer insulating layer ILD1. The third electrode C3 isconnected to the compensation TFT T3 and the first initialization TFT T4via the second contact hole 52 formed in the first to third gateinsulating layers GI1 to GI3, and the first interlayer insulating layerILD1. The “first data electrodes” as used in this embodiment refers tothe third electrode C3, the data line 16, and the first and secondcontact metals CM1 and CM2 which are formed of a first data line DAT1material. The first data line DAT1 material may include at least onemetal selected from the group of Al, Pt, Pd, Ag, Mg, Au, Ni, Nd, Ir, Cr,Li, Ca, Mo, Ti, W, and Cu.

According to some embodiments, the third electrode C3 does not overlapthe first storage capacitor Cst1. Therefore, the first and secondstorage capacitors Cst1 and Cst2 may be separated. Accordingly, thestorage capacity in each pixel may be substantially equal to the sum ofthe storage capacities of the first and second storage capacitors Cst1and Cst2.

A second interlayer insulating layer ILD2 is stacked on the entiresurface of the substrate 100 to cover the third electrode C3. The secondinterlayer insulating layer ILD2 may be formed of an organic insulatingmaterial, an inorganic insulating material, or have a multi-layerstructure in which organic and inorganic insulating materials arealternately formed. The second interlayer insulating layer ILD2insulates the first data electrode and second data electrodes andfunctions as a dielectric layer of the second storage capacitor Cst2.The “second data electrodes” will be described in detail below.

The fourth electrode C4 of the second storage capacitor Cst2 is formedon the second interlayer insulating layer ILD2. Although notillustrated, the initialization voltage line 20 that is connected to thefourth electrode C4, the driving voltage line 26, and the third contactmetal CM3, are simultaneously formed with the fourth electrode C4. Thefourth electrode C4 and the initialization voltage line 20 may beintegrally formed, and the fourth electrode C4 is connected to the firstand second initialization TFTs T4 and T7 by using the first contactmetal CM1 of the fourth contact hole 54. The “second data electrodes” asused in this embodiment refers to the fourth electrode C4, theinitialization voltage line 20, the driving voltage line 26, and thethird contact metal CM3 which are formed of a second data line DAT2material. The second data line DAT2 material may include at least onemetal selected from the group of Al, Pt, Pd, Ag, Mg, Au, Ni, Nd, Ir, Cr,Li, Ca, Mo, Ti, W, and Cu.

A protection layer PL is formed on the entire surface of the substrate100 to cover the fourth electrode C4. An anode electrode 111 is formedon the protection layer PL. The anode electrode 111 is connected to thesecond and third contact metals CM2 and CM3 formed in the sixth contacthole 56 via the via hole VIA, and thus is connected to the secondemission control drain electrode D6 and the second initialization sourceelectrode S7.

Although not illustrated in FIG. 3A, the anode electrode 111 of the OLEDis illustrated in FIG. 4 for the convenience of description. The OLEDincludes the anode electrode 111 and a cathode electrode (not shown)facing the anode electrode 111. An intermediate layer (not shown) thatincludes an organic emission layer is provided between the anodeelectrode 111 and the cathode electrode.

In FIGS. 3A and 4, the source and drain electrodes of the TFTs that arenot connected to other lines are formed on the same layer ascorresponding semiconductor layers. That is, the source and drainelectrodes of each of the TFTs may be selectively formed of polysiliconthat is doped with a doping material. However, the embodiments of thedescribed technology are not limited thereto. The TFTs according toanother embodiment include source and drain electrodes that are formedon a different layer from the corresponding semiconductor layers and areconnected to source and drain areas of the corresponding semiconductorlayers via a contact hole.

FIG. 5 is an equivalent circuit diagram of a pixel 1 of the OLED display1000, according to another embodiment.

The pixel 1 of the OLED display 1000 of the embodiment of FIG. 5includes a pixel circuit 2 that includes a plurality of TFTs T1 to T7and two storage capacitors, namely, first and second storage capacitorsCst1 and Cst2. The pixel 1 includes an OLED that receives a drivingcurrent Ioled via the pixel circuit 2 and thus emits light.

The plurality of TFTs T1 to T7 include a driving TFT T1, a switching TFTT2, a compensation TFT T3, a first initialization TFT T4, a firstemission control TFT T5, a second emission control TFT T6, and a secondinitialization TFT T7.

The pixel 1 includes a first scan line 14 that transmits a first scansignal Sn to the switching TFT T2 and the compensation TFT T3, a secondscan line 24 that transmits a second scan signal Sn−1 to the firstinitialization TFT T4, and a third scan line 34 that transmits a thirdscan signal Sn+1 to the second initialization TFT T7. The pixel 1 alsoincludes an emission control line 15 that transmits an emission controlsignal En to the first emission control TFT T5 and the second emissioncontrol TFT T6, a data line 16 that intersects the first scan line 14and transmits a data signal Dm, a driving voltage line 26 that transmitsthe first power voltage ELVDD, and an initialization voltage line 20that transmits an initialization voltage VINT to initialize the drivingTFT T1.

A gate electrode G1 of the driving TFT T1 is connected to a firstelectrode C1 of the first storage capacitor Cst1 and a third electrodeC3 of the second storage capacitor Cst2. A source electrode S1 of thedriving TFT T1 is connected to the driving voltage line 26 via the firstemission control TFT T5. A drain electrode D1 of the driving TFT T1 iselectrically connected to an anode electrode of the OLED via the secondemission control TFT T6. The driving TFT T1 receives the data signal Dmbased on a switching operation of the switching TFT T2 and supplies adriving current Ioled to the OLED.

A gate electrode G2 of the switching TFT T2 is connected to the firstscan line 14. A source electrode S2 of the switching TFT T2 is connectedto the data line 16. A drain electrode D2 of the switching TFT T2 isconnected to the source electrode S1 of the driving TFT T1 and to thedriving voltage line 26 via the first emission control TFT T5. Theswitching TFT T2 is turned on based on the first scan signal Sn receivedvia the first scan line 14 and performs a switching operation in whichthe data signal Dm received via the data line 16 is transmitted to thesource electrode S1 of the driving TFT T1.

A gate electrode G3 of the compensation TFT T3 is connected to the firstscan line 14. A source electrode S3 of the compensation TFT T3 isconnected to the drain electrode D1 of the driving TFT T1 and to theanode electrode of the OLED via the second emission control TFT T6. Adrain electrode D3 of the compensation TFT T3 is connected to the firstelectrode C1 of the first storage capacitor Cst1, the third electrode C3of the second storage capacitor Cst2, a drain electrode D4 of the firstinitialization TFT T4, and the gate electrode G1 of the driving TFT T1.The compensation TFT T3 is turned on based on the first scan signal Snreceived via the first scan line 14 and connects the driving TFT T1 toform a diode connection by connecting the gate electrode G1 and thedrain electrode D1 of the driving TFT T1.

A gate electrode G4 of the first initialization TFT T4 is connected tothe second scan line 24. A source electrode S4 of the firstinitialization TFT T4 is connected to the initialization voltage line20. The drain electrode D4 of the first initialization TFT T4 isconnected to the first electrode C1 of the first storage capacitor Cst1,the third electrode C3 of the second storage capacitor Cst2, the drainelectrode D3 of the compensation TFT T3, and the gate electrode G1 ofthe driving TFT T1. The first initialization TFT T4 is turned on basedon the second scan signal Sn−1 received via the second scan line 24 andperforms an initialization operation in which the initialization voltageVINT is transmitted to the gate electrode G1 of the driving TFT T1 andthus initializes a voltage of the gate electrode G1 of the driving TFTT1.

A gate electrode G5 of the first emission control TFT T5 is connected tothe emission control line 15. A source electrode S5 of the firstemission control TFT T5 is connected to the driving voltage line 26. Adrain electrode D5 of the first emission control TFT T5 is connected tothe source electrode S1 of the driving TFT T1 and the drain electrode D2of the switching TFT T2.

A gate electrode G6 of the second emission control TFT T6 is connectedto the emission control line 15. A source electrode S6 of the secondemission control TFT T6 is connected to the drain electrode D1 of thedriving TFT T1 and the source electrode S3 of the compensation TFT T3. Adrain electrode D6 of the second emission control TFT T6 is electricallyconnected to the anode electrode of the OLED. The first and secondemission control TFTs T5 and T6 are substantially simultaneously turnedon based on the emission control signal En received via the emissioncontrol line 15. Then, the first power voltage ELVDD is transmitted tothe OLED, and thus, the driving current Ioled flows in the OLED.

A gate electrode G7 of the second initialization TFT T7 is connected tothe third scan line 34. A source electrode S7 of the secondinitialization TFT T7 is connected to the anode electrode of the OLED. Adrain electrode D7 of the second initialization TFT T7 is connected tothe initialization voltage line 20. The second initialization TFT T7 isturned on based on the third scan signal Sn+1 received via the thirdscan line 34 and initializes the anode electrode of the OLED.

A second electrode C2 of the first storage capacitor Cst1 is connectedto the driving voltage line 26. The first electrode C1 of the firststorage capacitor Cst1 is connected to the gate electrode G1 of thedriving TFT T1, the third electrode C3 of the second storage capacitorCst2, the drain electrode D3 of the compensation TFT T3, and the drainelectrode D4 of the first initialization TFT T4.

The third electrode C3 of the second storage capacitor Cst2 is connectedto the gate electrode G1 of the driving TFT T1, the first electrode C1of the first storage capacitor Cst1, the drain electrode D3 of thecompensation TFT T3, and the drain electrode D4 of the firstinitialization TFT T4. A fourth electrode C4 of the second storagecapacitor Cst2 is connected to the second scan line 24.

The second power voltage ELVSS is applied to a cathode electrode of theOLED. The OLED displays an image by receiving the driving current Ioledfrom the driving TFT T1 and thus emits light.

FIGS. 6A and 6B are schematic plan views of the pixel 1 of FIG. 5. FIGS.6A and 6B each illustrate two adjacent pixels 1.

Referring FIG. 6A, the pixel 1 includes the driving TFT T1, theswitching TFT T2, the compensation TFT T3, the first initialization TFTT4, the first emission control TFT T5, the second emission control TFTT6, the second initialization TFT T7, the first storage capacitor Cst1,and the second storage capacitor Cst2. The OLED is omitted in FIG. 6A.

The driving TFT T1 includes a driving semiconductor layer A1, a drivinggate electrode G1, a driving source electrode S1, and a driving drainelectrode D1. The driving source electrode S1 is a driving source areain the driving semiconductor layer A1 which is doped with impurities andthe driving drain electrode D1 is a driving drain area in the drivingsemiconductor layer A1 which is doped with impurities. The driving gateelectrode G1 is connected to the first electrode C1 of the first storagecapacitor Cst1, the third electrode C3 of the second storage capacitorCst2, a compensation drain electrode D3 of the compensation TFT T3, anda first initialization drain electrode D4 of the first initializationTFT T4. In particular, the driving gate electrode G1 and the firstelectrode C1 are integrally formed on the same layer. The driving gateelectrode G1 and the third electrode C3 are connected by using a firstcontact metal CM1 formed in a first contact hole 51. The driving gateelectrode G1, the compensation drain electrode D3, and the firstinitialization drain electrode D4 are connected by using the firstcontact metal CM1 formed in each of the first contact hole 51 and asecond contact hole 52.

The switching TFT T2 includes a switching semiconductor layer A2, aswitching gate electrode G2, a switching source electrode S2, and aswitching drain electrode D2. The switching source electrode S2 is aswitching source area in the switching semiconductor layer A2 which isdoped with impurities and the switching drain electrode D2 is aswitching drain area in the switching semiconductor layer A2 which isdoped with impurities. The switching source electrode S2 is connected tothe data line 16 via a third contact hole 53. The switching drainelectrode D2 is connected to the driving TFT T1 and the first emissioncontrol TFT T5. The switching gate electrode G2 is formed as a portionof the first scan line 14.

The compensation TFT T3 includes a compensation semiconductor layer A3,a compensation gate electrode G3, a compensation source electrode S3,and the compensation drain electrode D3. The compensation sourceelectrode S3 is a compensation source area in the compensationsemiconductor layer A3 which is doped with impurities and thecompensation drain electrode D3 is a compensation drain area in thecompensation semiconductor layer A3 which is doped with impurities. Thecompensation gate electrode G3 forms a dual gate electrode by using aportion of the first scan line 14 and a portion of a line that protrudesand extends from the first scan line 14, and thus prevents leakagecurrent.

The first initialization TFT T4 includes a first initializationsemiconductor layer A4, a first initialization gate electrode G4, afirst initialization source electrode S4, and the first initializationdrain electrode D4. The first initialization source electrode S4 is afirst initialization source area in the first initializationsemiconductor layer A4 which is doped with impurities, and the firstinitialization drain electrode D4 is a first initialization drain areain the first initialization semiconductor layer A4 which is doped withimpurities. The first initialization source electrode S4 may beconnected to the compensation TFT T3. The first initialization drainelectrode D4 may be connected to a fourth electrode C4 of the secondstorage capacitor Cst2 via the second contact hole 52, and to thedriving gate electrode G1 and the first electrode C1 by using the firstcontact metal CM1 formed in the first and second contact holes 51 and52. The first initialization gate electrode G4 is formed by a portion ofthe second scan line 24. A dual gate electrode is formed by overlappingthe first initialization semiconductor layer A4 and the firstinitialization gate electrode G4 twice.

The first emission control TFT T5 includes a first emission controlsemiconductor layer A5, a first emission control gate electrode G5, afirst emission control source electrode S5, and a first emission controldrain electrode D5. The first emission control source electrode S5 is afirst emission control source area in the first emission controlsemiconductor layer A5 which is doped with impurities and the firstemission control drain electrode D5 is a first emission control drainarea in the first emission control semiconductor layer A5 which is dopedwith impurities. The first emission control source electrode S5 may beconnected to the driving voltage line 26 via a fourth contact hole 54.The first emission control gate electrode G5 is formed as a portion ofthe emission control line 15.

The second emission control TFT T6 includes a second emission controlsemiconductor layer A6, a second emission control gate electrode G6, asecond emission control source electrode S6, and a second emissioncontrol drain electrode D6. The second emission control source electrodeS6 is a second emission control source area in the second emissioncontrol semiconductor layer A6 which is doped with impurities and thesecond emission control drain electrode D6 is a second emission controldrain area in the second emission control semiconductor layer A6 whichis doped with impurities. The second emission control drain electrode D6is connected to the anode electrode of the OLED via a second contactmetal CM2 formed through a fifth contact hole 55 and a via hole VIA,both of which are filled with the second contact metal CM2. The secondemission control gate electrode G6 is formed by a portion of theemission control line 15.

The second initialization TFT T7 includes a second initializationsemiconductor layer A7, a second initialization gate electrode G7, asecond initialization source electrode S7, and a second initializationdrain electrode D7. The second initialization source electrode S7 is asecond initialization source area in the second initializationsemiconductor layer A7 which is doped with impurities and the secondinitialization drain electrode D7 is a second initialization drain areain the second initialization semiconductor layer A7 which is doped withimpurities. The second initialization source electrode S7 may beconnected to the initialization voltage line 20 via a sixth contact hole56, and the second initialization drain electrode D7 may be connected tothe anode electrode of the OLED by using the second contact metal CM2formed through the fifth contact hole 55 and the via hole VIA, both ofwhich are filled with the second contact metal CM2. The secondinitialization gate electrode G7 is formed as a portion of the thirdscan line 34.

The first electrode C1 of the first storage capacitor Cst1 is directlyconnected to the driving gate electrode G1 and may be connected to thethird electrode C3 of the second storage capacitor Cst2 via the firstcontact metal CM1 formed in the first contact hole 51. Also, the firstelectrode C1 of the first storage capacitor Cst1 is connected to thefirst initialization TFT T4 and the compensation TFT T3 by using thefirst contact metal CM1 formed in the first and second contact holes 51and 52. The first electrode C1 may have a floating electrode form andmay overlap the driving semiconductor layer A1.

Although the second electrode C2 of the first storage capacitor Cst1overlaps the first electrode C1, the second electrode C2 does notcompletely cover the first electrode C1. The second electrode C2includes an opening OP that exposes a portion of the first electrode C1.The opening OP may be the first contact hole 51. The second electrodesC2 that are formed in the two adjacent pixels 1 may be connected to eachother. The driving voltage line 26 may be connected to the center of thesecond electrodes C2 that are formed in the two adjacent pixels 1 via aseventh contact hole 57, and thus may substantially simultaneouslytransmit the first power voltage ELVDD to the two adjacent pixels 1.That is, the two adjacent pixels 1 receive the driving voltage ELVDDfrom a single driving voltage line 26 by using the second electrodes C2that are formed in the two adjacent pixels 1.

The third electrode C3 of the second storage capacitor Cst2 is connectedto the first electrode C1 of the first storage capacitor Cst1 and thedriving gate electrode G1 via the first contact metal CM1 formed in thefirst contact hole 51. Also, the third electrode C3 is connected to thecompensation drain electrode D3 and the first initialization drainelectrode D4 via the second contact hole 52. The third electrode C3 andthe data line 16 may be formed on the same layer. The third electrode C3is separated from the first storage capacitor Cst1 and does not overlapthe first storage capacitor Cst1. Therefore, as illustrated in FIG. 6B,the second storage capacitor Cst2 may be separated from the firststorage capacitor Cst1 and the second storage capacitor Cst2 may have aseparate storage capacity different from the storage capacity of thefirst storage capacitor Cst1. Accordingly, it is possible to solve theproblem of decreasing storage capacity in pixels that may occur due toan increase in OLED display resolution. Thus, the storage capacity ineach of the pixels may be maintained with a value that is similar tothat of the standard OLED display.

The fourth electrode C4 of the second storage capacitor Cst2 overlapsthe third electrode C3. The fourth electrode C4 and the second scan line24 are formed on the same layer. In particular, the fourth electrode C4is formed by a portion of the second scan line 24. Since the second scanline 24 forms the first initialization gate electrode G4, the fourthelectrode C4 may be connected to the first initialization TFT T4.

The first to third scan lines 14, 24, and 34 and the emission controlline 15 are all formed on the same layer and extend in the seconddirection. The first to third scan lines 14, 24, and 34 and the emissioncontrol line 15 may be formed on the same layer as the first electrodeC1 of the first storage capacitor Cst1 and the fourth electrode C4 ofthe second storage capacitor Cst2.

The data line 16, the driving voltage line 26, and the initializationvoltage line 20 are formed on the same layer as the third electrode C3of the second storage capacitor Cst2 and extend in the first direction.

The two adjacent pixels 1 share the driving voltage line 26. Inparticular, the driving voltage line 26 is formed between the twoadjacent pixels 1 and extends in the first direction. The drivingvoltage line 26 is connected to the first emission control TFT T5, whichis included in each of the two adjacent pixels 1, via the fifth contacthole 55, and to the second electrode C2 of the first storage capacitorCst1, which is commonly included in the two adjacent pixels 1, via theseventh contact hole 57. According to the embodiment of FIGS. 6A and 6B,the two adjacent pixels 1 share the driving voltage line 26, and thusthe two adjacent pixels 1 may form a substantially symmetrical structureabout the driving voltage line 26. Accordingly, the number of drivingvoltage lines 26 may be reduced, and thus, it is possible to obtain moredesign space due to this reduction.

FIG. 7 is a cross-sectional view taken along lines C-C′, D-D′, and E-E′of FIG. 6A. FIG. 7 illustrates the driving TFT T1 and the secondemission control TFT T6, the first storage capacitor Cst1, and thesecond storage capacitor Cst2.

Among elements such as lines, electrodes, and semiconductor layers, in across-section taken along lines A-A′ and B-B′, elements that have littlerelation to the driving TFT T1, the second emission control TFT T6, thefirst storage capacitor Cst1, and the second storage capacitor Cst2 areomitted in FIG. 7 so as to clarify the features of the embodimentillustrated in FIGS. 6A and 6B. Therefore, FIG. 7 may be different froman actual cross-sectional view taken along lines C-C′, D-D′, and E-E′ ofFIG. 6A.

Referring to FIG. 7, a buffer layer 101 is formed on a substrate 100.The buffer layer 101 functions as a barrier layer and/or blocking layerfor preventing impurity ions from spreading, blocking penetration ofmoisture or external air, and planarizing the surface of the substrate100.

The driving semiconductor layer A1 of the driving TFT T1 and the secondemission control semiconductor layer A6 of the second emission controlTFT T6 are formed on the buffer layer 101. The driving and secondemission control semiconductor layers A1 and A6, which are formed ofpolysilicon, include a channel area that is not doped with impurities,and a source area and a drain area which are respectively formed on bothsides of the channel area and doped with impurities. In the embodimentof FIG. 7, the impurities differ according to the type of TFT, and maybe N-type impurities or P-type impurities. Although not illustrated, thedriving semiconductor layer A1 and the second emission controlsemiconductor layer A6 may be simultaneously formed while beingconnected to the switching semiconductor layer A2 of the switching TFTT2, the compensation semiconductor layer A3 of the compensation TFT T3,the first initialization semiconductor layer A4 of the firstinitialization TFT T4, the second initialization semiconductor layer A7of the second initialization TFT T7, and the first emission controlsemiconductor layer A5 of the first emission control TFT T5.

A first gate insulating layer GI1 is formed on the entire surface of thesubstrate 100 to cover the driving and second emission controlsemiconductor layers A1 and A6. The first gate insulating layer GI1 maybe a single layer or layers formed of an inorganic material such assilicon oxide or silicon nitride. The first gate insulating layer GI1insulates the semiconductor layers from the first gate electrodes.According to an embodiment, the first gate insulating layer GI1 isthicker than a second gate insulating layer GI2 which will be describedbelow. The first gate insulating layer GI1 insulates the semiconductorlayers of the driving TFT T1, the switching TFT T2, the compensation TFTT3, the first initialization TFT T4, the first emission control TFT T5,the second emission control TFT T6, and the second initialization TFT T7from the first gate electrodes. The “first gate electrodes” will bedescribed in detail below. When the first gate insulating layer GI1 isthick, a parasitic capacitance formed between the semiconductor layersand the first gate electrodes may be reduced, and thus, stains may bereduced or removed from images displayed by the OLED display. Inaddition, in the driving TFT T1, the parasitic capacitance between thedriving semiconductor layer A1 and the driving gate electrode G1 isreduced, and thus, a gate voltage Vgs, which is applied to the drivinggate electrode G1, has a wide driving range. Accordingly, a magnitude ofthe gate voltage Vgs may be changed so that light emitted from the OLEDhas more various gray scales.

The second emission control gate electrode G6 of the second emissioncontrol TFT T6, the driving gate electrode G1 of the driving TFT T1, thefirst electrode C1 of the first storage capacitor Cst1, and the fourthelectrode C4 of the second storage capacitor Cst2 are formed on thefirst gate insulating layer GI1. Also, although not illustrated, theswitching gate electrode G2 of the switching TFT T2, the compensationgate electrode G3 of the compensation TFT T3, the first initializationgate electrode G4 of the first initialization TFT T4, the secondinitialization gate electrode G7 of the second initialization TFT T7,and the first emission control gate electrode G5 of the first emissioncontrol TFT T5 are simultaneously formed with the second emissioncontrol gate electrode G6, the driving gate electrode G1, the firstelectrode C1, and the fourth electrode C4. The “first gate electrodes”as used in this embodiment refers to the driving gate electrode G1, theswitching gate electrode G2, the compensation gate electrode G3, thefirst initialization gate electrode G4, the first emission control gateelectrode G5, the second emission control gate electrode G6, the secondinitialization gate electrode G7, the first electrode C1, and the fourthelectrode C4 which are formed of a first gate line GL1 material.

The switching gate electrode G2, the compensation gate electrode G3, thefirst initialization gate electrode G4, the first emission control gateelectrode G5, the second emission control gate electrode G6, and thesecond initialization gate electrode G7 are defined by semiconductorlayers that overlap the first to third scan lines 14, 24, and 34 and theemission control line 15. Also, the fourth electrode C4 is a portion ofthe second scan line 24. Therefore, a process of forming the switchinggate electrode G2, the compensation gate electrode G3, the firstinitialization gate electrode G4, the first emission control gateelectrode G5, the second emission control gate electrode G6, the secondinitialization gate electrode G7, and the fourth electrode C4 is thus aprocess of forming the first to third scan lines 14, 24, and 34 and theemission control line 15. The driving gate electrode G1 and the firstelectrode C1 are integrally formed. The first gate line GL1 material mayinclude at least one metal selected from the group of Al, Pt, Pd, Ag,Mg, Au, Ni, Nd, Ir, Cr, Li, Ca, Mo, Ti, W, and Cu.

According to an embodiment, the first storage capacitor Cst1 overlapsthe driving TFT T1. In particular, since the driving gate electrode G1and the first electrode C1 are integrally formed in this embodiment, thefirst storage capacitor Cst1 has to overlap the driving TFT T1. However,when the entire size of the driving TFT T1, more particularly, a size ofthe driving gate electrode G1, is reduced in order to manufacture ahigh-resolution OLED display, the size of the first electrode C1 is alsoreduced, and thus, the storage capacity of the first storage capacitorCst1 is reduced according to Equation 1 below. In Equation 1, “C” is thestorage capacity of a storage capacitor, “c” is the dielectric constant,“A” is the overlapping area of the two electrodes of the storagecapacitor, and “d” is the distance between the overlapping electrodes.

$\begin{matrix}{C = {ɛ\frac{A}{d}}} & \lbrack {{Equation}\mspace{14mu} 1} \rbrack\end{matrix}$

A substantially uniform storage capacity should to be maintained in apixel so that gray scales are displayed by the OLED display withouterror. Therefore, a design for maintaining or supplementing the reducedstorage capacity of the first storage capacitor Cst1 is necessary.According to an embodiment, the second storage capacitor Cst2 isseparately formed from the first storage capacitor Cst1 so as tomaintain or supplement the total storage capacity of the capacitors ineach pixel.

The second gate insulating layer GI2 may be stacked on the entiresurface of the substrate 100 to cover the first gate electrodes. Thesecond gate insulating layer GI2 may be a single layer or layers formedof an inorganic material such as silicon oxide or silicon nitride. Thesecond gate insulating layer GI2 insulates the first and second gateelectrodes from each other. The “second gate electrode” will bedescribed in detail below. Also, the second gate insulating layer GI2functions as a dielectric layer of the first storage capacitor Cst1. Inorder to increase the storage capacity of the first storage capacitorCst1 based on Equation 1, the second gate insulating layer GI2 isthinner than the first gate insulating layer GI1.

The second electrode C2 of the first storage capacitor Cst1 is formed onthe second gate insulating layer GI2. The second electrode C2 overlapsthe first electrode C1. However, the second electrode C2 includes theopening OP that exposes a portion of the first electrode C1. The openingOP may be the first contact hole 51. The first electrode C1 may beconnected to the third electrode C3 via the first contact hole 51. The“second gate electrode” as used in this embodiment refers to the secondelectrode C2 that is formed of a second gate line GL2 material. Similarto the first gate line GL1 material, the second gate line GL2 materialmay also include at least one metal selected from the group of Al, Pt,Pd, Ag, Mg, Au, Ni, Nd, Ir, Cr, Li, Ca, Mo, Ti, W, and Cu.

The first interlayer insulating layer ILD1 is formed on the entiresurface of the substrate 100 to cover the second electrode C2 of thefirst storage capacitor Cst1. The first interlayer insulating layer ILD1may be formed of an organic insulating material, an inorganic insulatingmaterial, or have a multi-layer structure in which organic and inorganicinsulating materials are alternately formed. For example, the inorganicinsulating material may include a metal oxide or a metal nitride, suchas silicon oxide (SiO₂), silicon nitride (SiN_(x)), silicon oxynitride(SiON), aluminum oxide (Al₂O₃), titanium oxide (TiO₂), tantalum oxide(Ta₂O₅), hafnium oxide (HfO₂), or zinc oxide (ZnO₂). The firstinterlayer insulating layer ILD1 insulates the second electrode C2 fromthe first electrode C1. Also, the first interlayer insulating layerILD1, together with the second gate insulating layer GI2, functions as adielectric layer of the second storage capacitor Cst2.

The third electrode C3 of the second storage capacitor Cst2 is formed onthe first interlayer insulating layer ILD1. The third electrode C3overlaps the fourth electrode C4 to form the second storage capacitorCst2. Although not illustrated, the data line 16, the driving voltageline 26, and the first and second contact metals CM1 and CM2 aresimultaneously formed with the third electrode C3. The third electrodeC3 is connected to the first electrode C1 via the first contact hole 51formed in the second gate insulating layer GI2 and the first interlayerinsulating layer ILD1. The third electrode C3 is connected to thecompensation TFT T3 and the first initialization TFT T4 via the secondcontact hole 52 formed in the first and second gate insulating layersGI1 and GI2, and the first interlayer insulating layer ILD1. “Dataelectrodes” as used in this embodiment refers to the third electrode C3,the data line 16, the driving voltage line 26, and the first and secondcontact metals CM1 and CM2 which are formed of a data line DAT material.The data line DAT material may include at least one metal selected fromthe group of Al, Pt, Pd, Ag, Mg, Au, Ni, Nd, Ir, Cr, Li, Ca, Mo, Ti, W,and Cu.

According to at least one embodiment, the second storage capacitor Cst2does not overlap the first storage capacitor Cst1. Therefore, as shownin FIG. 6B, the first and second storage capacitors Cst1 and Cst2 may beseparated. Accordingly, the storage capacity of each pixel may besubstantially equal to the sum of storage capacities of the first andsecond storage capacitors Cst1 and Cst2.

A protection layer PL is formed on the entire surface of the substrate100 to cover the third electrode C3. An anode electrode 111 is formed onthe protection layer PL. The anode electrode 111 is connected to thesecond contact metal CM2 formed in the fifth contact hole 55 via the viahole VIA, and thus is connected to the second emission control drainelectrode D6 and the second initialization source electrode S7.

Although not illustrated in FIG. 6A, the anode electrode 111 of the OLEDis illustrated in FIG. 7 for the convenience of description. The OLEDincludes the anode electrode 111 and a cathode electrode facing theanode electrode 111. An intermediate layer that includes an organicemission layer is provided between the anode electrode 111 and thecathode electrode.

In FIGS. 6A and 7, from among the source and drain electrodes of theTFTs, source and drain electrodes that are not connected to other linesare formed on the same layer as corresponding semiconductor layers. Thatis, the source and drain electrodes of each of the TFTs may beselectively formed of polysilicon that is doped with a doping material.However, described technology is not limited thereto. A TFT according toanother embodiment includes source and drain electrodes that are formedon a different layer from corresponding semiconductor layers and beconnected to source and drain areas of the corresponding semiconductorlayers via a contact hole.

As described above, according to at least one embodiment, ahigh-resolution OLED display maintains and supplements the capacitanceof the pixels even when a plurality of pixels are formed in a limitedspace.

It should be understood that the exemplary embodiments described hereinshould be considered in a descriptive sense only and not for purposes oflimitation. Descriptions of features or aspects within each embodimentshould typically be considered as available for other similar featuresor aspects in other embodiments.

While one or more embodiments of the inventive technology have beendescribed with reference to the figures, it will be understood by thoseof ordinary skill in the art that various changes in form and detailsmay be made therein without departing from the spirit and scope of thepresent invention as defined by the following claims.

What is claimed is:
 1. An organic light-emitting diode (OLED) display,comprising: a plurality of pixels, wherein each of the pixels comprises:a driving thin film transistor (TFT) formed over a substrate andincluding a driving gate electrode; a first storage capacitor comprisinga first electrode and a second electrode, wherein the first electrode iselectrically connected to the driving gate electrode and wherein thesecond electrode is formed over the first electrode and electricallyinsulated from the first electrode; a second storage capacitorcomprising a third electrode and a fourth electrode, wherein the secondstorage capacitor does not overlap the first storage capacitor, whereinthe third electrode is electrically connected to the first electrode,and wherein the fourth electrode is: i) formed under the thirdelectrode, ii) electrically insulated from the third electrode, and ii)formed on the same layer as the first electrode, wherein the thirdelectrode of the second storage capacitor and the second electrode ofthe first storage capacitor are formed on and over different layers; andan interlayer insulating layer having bottom and top surfaces, whereinthe bottom surface faces the TFT, wherein the bottom surface of theinterlayer insulating layer covers the second electrode of the firststorage capacitor, and wherein the third electrode of the second storagecapacitor is formed on the top surface of the interlayer insulatinglayer.
 2. The OLED display of claim 1, wherein each of the driving gateelectrodes is integrally formed with the corresponding first electrode.3. The OLED display of claim 1, wherein each of the driving TFTs furthercomprises a driving semiconductor layer formed under the driving gateelectrode, and wherein each of the driving semiconductor layers iselectrically insulated from the corresponding driving gate electrode viaa first gate insulating layer.
 4. The OLED display of claim 3, whereineach of the pixels further comprises an OLED and an emission control TFTformed between the driving TFT and the OLED and wherein each of theemission control TFTs comprises: an emission control semiconductor layerformed on the same layer as the driving semiconductor layer; and anemission control gate electrode electrically insulated from the emissioncontrol semiconductor layer via the first gate insulating layer, whereinthe emission control gate electrode is formed on the same layer as thedriving gate electrode.
 5. The OLED display of claim 4, furthercomprising a second gate insulating layer formed between the first andsecond electrodes.
 6. The OLED display of claim 5, wherein theinterlayer insulating layer is formed between the third and fourthelectrodes.
 7. The OLED display of claim 4, wherein each of the secondelectrodes is electrically connected to a corresponding driving voltageline configured to transmit a driving voltage for driving thecorresponding OLED.
 8. The OLED display of claim 4, further comprising aprotection layer formed over the third electrode, wherein each of theOLEDs comprises: an anode electrode formed over the protection layer; acathode electrode facing the anode electrode; and an intermediate layerinterposed between the anode and cathode electrodes.
 9. The OLED displayof claim 1, wherein each pixel further comprises an initialization TFTformed between the driving TFT and an initialization voltage lineconfigured to transmit an initialization voltage, wherein each of theinitialization TFTs is configured to be turned on by a correspondingscan signal so as to transmit the initialization voltage to thecorresponding driving gate electrode.
 10. The OLED display of claim 9,wherein the fourth electrode is electrically connected to a scan lineconfigured to transmit the scan signal.